The present invention relates generally to radiographic and tomographic imaging, and, more particularly, to estimating and reducing scatter in digital radiographic and tomographic imaging and to an improved digital X-ray detector used for same.
A typical prior art radiographic or computed tomography (CT) imaging system 10 of the so-called third generation is shown in FIG. 1. Imaging systems of the type disclosed in FIG. 1 are described in further detail in Principles of Computerized Tomographic Imaging, by Avinash C. Kak and Malcolm Slaney, IEEE Press, 1988. As shown in FIG. 1, the imaging system 10 includes a source 12, such as an x-ray source, transmitting primary signals to an object 14, such as a patient, positioned on a support 16, such as a table. Some of the primary signals pass through the object 14 and the support 16, and are detected by detector array 18. Detection of the primary signals by detector array 18 is controlled by data acquisition component 19.
A characteristic of a third-generation CT imaging system is that the source 12 and the detector array 18 containing collimating plates are focally aligned with each other and are both controlled by a common controller 20 to move in tandem with each other while maintaining their established focal alignment. Focal alignment means that the collimating plates of the detector array 18 point toward the source 12. The controller 20 typically controls on/off states and motion of the source 12 and the detector array 18, based upon instructions issued by the CT system computer 22. The CT system computer 22 also controls the data acquisition component 19.
Once the x-ray signals are detected, data acquisition component 19 converts the detected signals into digital data supplied to the CT system computer 22. The CT system computer 22 then processes according to well-known techniques the digital data, stores the processed digital data in system memory 24, and displays the processed digital data on display 26.
When high-resolution digital area detectors are used in radiographic and tomographic imaging applications, one concern that commonly arises is the problem of scatter corruption of the primary signal. Scatter, or scatter corruption, reduces low-contrast detectability and resolution in both radiographic images and reconstructed tomographic images. Further, scatter signals can cause streaks between highly attenuating objects in reconstructed tomographic images, thus masking the shape of the objects 14. Unlike with lower resolution linear detectors, it is impossible to collimate appropriately each detector element in an area detector since the dimension of individual detector elements can be an order of magnitude smaller than their linear detector counterparts. As a result, it is necessary to devise methods or schemes to either reduce scatter or correct for scatter.
An approach of the prior art in the field of radiography has been to use a collimating grid positioned over the detector. Although this grid covers part of the detector, it is continuously moved during the x-ray exposure interval so as not to produce discernible artifacts.
Another approach offered in the prior art has been to estimate scatter in regions of the imager that are not shadowed by the patient. A constant value of scatter is then subtracted from all pixels in the image. Although this approach has some benefit for radiographic applications, it usually produces severe artifacts in tomographic applications. In general, using area detectors for tomographic applications is a fairly new technology; hence, no standard methods to reduce/correct scatter have been devised. As used herein, xe2x80x9carea detectorsxe2x80x9d generally refer to detectors having rows and columns of pixels that are connected together to provide readout by rows and columns of the pixels in the array, and typically implies a large number (e.g., more than 5 rows, and commonly an array of a size 1000xc3x971000 pixels or detector elements) of pixels in the array; multi-row detectors typically refer to a small number (e.g., 5 or less) pixel rows that are arranged to provide an imaging signal, and a xe2x80x9csingle rowxe2x80x9d detector refers to one row of pixels disposed to provide the imaging signal.
In most x-ray imaging systems using area detectors, the x-ray detector that is used to measure the intensity of the x-ray beam, or primary signal, that remains after passing through the object (or patient) does not completely absorb the remaining x-ray flux from the x-ray beam. As a result, a slab of attenuating material called a beam-stop 28 is usually placed directly behind the detector 18 panel, as shown in FIG. 2. The use of a beam-stop 28 is discussed in xe2x80x9cMeasurement of Scatter Fractions in Clinical Bedside Radiography,xe2x80x9d Radiology 1992, 183:857-861. The slab of attenuating material (or beam-stop) 28 is usually made from an x-ray absorbing material such as lead or tungsten and is used to reduce the residual x-ray intensity that is not detected by the area detector 18 to a near zero value. Since a part of the x-ray flux is not absorbed by the area detector 18, some of the dose applied to the patient 14 is not used for diagnostic purposes, a health concern for radiologists.
Since x-ray scatter, resulting from the interaction of the primary x-ray beam P1 (as shown in FIG. 2) with the object 14 within the imaging system 10, may be a significant fraction of the detected x-ray signal in the area detector 18, a method and apparatus for estimating and reducing the scatter signal would be useful. If x-ray data collected contain a significant amount of scatter, computed tomography (or CT) reconstructions of the object will contain noticeable artifacts that will limit their utility for diagnostic purposes.
It is known in the art that hardware collimation significantly reduces the scatter signal measured with linear detector arrays 18. A detector array 18 comprises cells 30. In general, for these types of detector arrays 18 in a third-generation CT imaging machine 10, each cell 30 of the detector array 18 includes individual detector elements 18-n and may also include collimator plates 32 as shown in FIG. 3. A typical area detector array 18 shown in FIGS. 1-3 includes, for example, 1Kxc3x971K (1,024xc3x971,024) of detector elements 18-n or cells 30 detecting the x-ray beam. In the detector cell 30 shown in FIG. 3, collimator plates 32 are included. However, collimator plates 32 are not necessarily included in other detector cells 30 of area detector arrays 18. As shown in FIG. 3, a collimator plate 32, made of thin lead or tungsten and which is focally aligned to the x-ray source 12, is placed between each detector element 18-n in detector array 18; these plates 32 attenuate the scatter signal while allowing the primary signal to be detected as desired. The dimensions of the collimator plates 32 affect the amount of scatter that is rejected by the imaging geometry of the imaging system 10. For area detector technology, it is not possible to collimate each detector element 18-n (also referred to as cell 30) in area detector array 18 because of increased resolution of individual detector elements 18-n or cells 30 used in area detector array 18. The thickness of the collimator plates 32 required to appropriately attenuate the scattered radiation would be such that the plates 32 would cover most, if not all, of the active area of the individual detector elements 18-n or cells 30 in area detector array 18. Therefore, other methods are needed to appropriately characterize and correct for scatter.
The present invention provides an x-ray imaging system and method which images an object by transmitting primary signals through the object. In the present invention, a collimator is placed between two detectors of the x-ray imaging system. The collimator reduces respective scatter components of total signals measured in one of the detectors; the total signals comprise a transmitted primary component and a scattered signal component. More particularly, a first detector detects the total signals, the collimator collimates the primary signals of the total signals focally aligned with the collimator and traveling through the first detector, and a second detector detects the collimated, primary signals. The x-ray imaging system reduces the scatter components of the total signals detected by the first detector based on the detected, collimated signals detected by the second detector and the corresponding, detected, total signals detected by the first detector.
The present invention also provides a scatter estimation and reduction method of an x-ray imaging system transmitting primary signals detected by one of the detectors of the x-ray imaging system. More particularly, the method of the present invention detects by a first detector total signals, then collimates, by a collimator placed between the first detector and a second detector, components of the total signals passing through the first detector, and, subsequently, detects by the second detector the collimated signals. The method of the present invention then reduces the scatter components of the total signals detected by the first detector based on the detected, collimated signals detected by the second detector and the corresponding, detected, total signals detected by the first detector.
The present invention includes a scatter estimation and reduction software program executed by a processor and implementing the above-mentioned method of the present invention.